I was thinking about space as a measured and marked series of boundary lines to denote ownership and governance. The problem is that I'm having trouble seeing that as useful. If there's no particular immovable assets or resources there, why would you spend effort occupying it? Consider how we have vast oceans with 7.5 billion people on earth and over 200 sovereign nations. Even still the most aggressive occupation actions are localized to shorelines and fossil fuel resources.

Let's consider Star Trek's take: space is parceled and occupied, and it's a major offense to cross some boundaries. Now let's consider Dune's take: space is a vast waste with small picks of usefulness spread so thin, it's like a handful of rice spread out in the sea. I understand space travel in Dune is of the "space-time folding" type and is thus instantaneous, while Star Trek is linear FTL "warp". Despite this, I'm having trouble believing any spacing society would spend much effort protecting massive voids between the bits that actually matter. How can the Star Trek style "space occupation" be explained as necessary, desirable, or even possible?

$\begingroup$@L.Dutch Star Trek and Dune are just examples to help get my point across. How can the Star Trek style "space occupation" be explained as necessary, desirable, or even possible? Following the Star Trek example, it's just taken as a given. Federation space ... Klingon space ... trespassers ... ensuing space battles ...$\endgroup$
– fredsbendAug 8 at 7:05

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$\begingroup$Boundaries work the same way as they do in the real world. You patrol them to make sure the enemy isn't just sitting there with an entire army waiting to attack you. The larger your boundaries expand past your actual resources/occupied locations, the more time you have to prepare for an incoming attack (it takes time to travel). It also costs more resources to actively monitor the area. So basically its like a fence. Anyone past the fence, I threaten to shoot immediately. That way, even if they wait on the other side, I have some time before they reach the front door to prepare.$\endgroup$
– ShadowzeeAug 8 at 7:52

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$\begingroup$The time element is exactly why space territory matters in Star Trek but not in Dune. Star Trek FTL still takes time, while Dune FTL is instant. There's no value to a buffer zone if the enemy can just appear at your doorstep with no warning from any distance. Note that in cases where travel is essentially instant in Star Trek (e.g. the wormhole in DS9), they deliberately arrange to have extra defenses at such locations.$\endgroup$
– Darrel HoffmanAug 8 at 20:08

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$\begingroup$Something that kept occurring to me as I read answers was that FTL travel with normal sub-light communication could have interesting results, like a starship only seeing what is behind or a starbase not being able to detect an FTL attacker. You should either solve this problem or use it as a plot point.$\endgroup$
– GraultAug 9 at 18:32

8 Answers
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Its less about occupying a space and more about setting a boundary in all senses of the word.

"If you cross this boundary then your ships would get a tactical advantage in scouting out potential weaknesses and might be in a better position to attack some of my colonies. So if you cross it we will assume that action is happening and retaliate accordingly".

This is why countries might claim patches of sea such as straits or claim deserted regions, it forms a buffer so their forces have depth of territory to give up when pressured by superior forces and gives more time to formulate a plan or warn off potential enemies before they become a direct threat to trade or military/civilian areas.

You can see this in Star Trek. They don't just have a border but a neutral zone as well. It takes time to cross the neutral zone, time in which scanners can potentially detect them and prevents both sides from seeing clearly what the other team is doing along their border. And whenever someone is detected in the neutral zone then they have time to engage in diplomacy, "return or we assume hostile action".

You might think of it as the outer-space equivalent of the Treaties of Tordesillas and Zaragoza.

The situation is most space opera is similar to the situation of Earth in 15th and early 16th centuries, at least from the point of view of the European nations: you generally know where the big stuff is, but there's still a lot that's unknown. Spain and Portugal divided up the planet between them by drawing borders and agreeing that everything on one side belonged to Spain, on the other to Portugal, with the aim they could then focus on exploring and developing everything on their given side without worrying about claim-jumping from the other.

So, to use the Star Trek example, the borders between the Federation and the Klingons is a 3D boundary where they've agreed that they have exclusive authority on their respective sides, and that they have claim to anything that happens to be found, discovered, or developed on said sides. In that setting they're still discovering new planets, new resources, ancient derelict starships, new intelligent species and so on, just as the Spanish and Portugese were still discovering new islands and territories to settle, new people and new resources to exploit, and so on.

How they exert that authority internally is an independent question, but both sides have agreed that if it's on the other side of that boundary, it's a Not My Problem situation, so if the Klingons want to conquer newly discovered planets, the Federation may complain on moral grounds if they find out, but they don't rush to that planet's rescue because it's officially Not Their Problem. If the Federation wants to isolate a pre-warp planet and let them develop naturally and eventually make contact and offer membership, the Klingons may find that funny, but whatever, it's Not Their Problem.

Again, this makes sense in the Star Trek context because in that setting there's still a lot of stuff that isn't explored even relatively nearby, and the method of travel allows you to go anywhere within it, so you never know what you might stumble on in interstellar space. Even if you're travelling FTL in that setting, you can still scan and monitor the "normal" space around you.

Dune, as you point out, is different because the method of travel is different. When you're dealing with jumps or hyperspace or wormholes some other kind of interstellar travel such that you're never likely to stumble across anything in normal space between travel points, then there's little interest or point in claiming it, so "borders" in it make little sense aside from being drawn on maps simply as a convenient visual way to separate systems.

From a military point of view, this differentiation makes strategic sense as well. In Star Trek, the technology is such that you can monitor space and detect starships (cloaking aside) in between solar systems. Thus, a clearly defined boundary is sort of necessary. Then Starfleet and the Klingons can merrily cruise on their own sides without worrying that their very presence could be considered an act of war because the other side could detect them.

On the other hand, consider a setting that uses jumpoints. Let's use Pournelle's CoDominium, or the Starfire settings as an example: ships pop from one system to another and aren't in between, so even leaving the instantaneous nature of the travel aside, there's no way to detect a ship travelling between system. For practical purposes, there's nothing between "here" and "there", so there's no point in trying to define or defend a border; you defend systems.

Less advanced tech version: if you have a city that allows easy travel across it, at any place and any time, then street gangs would need clearly defined boundaries between them to differentiate, and those boundaries would need to be patrolled and monitored, and possible defended. If, on the other hand, you had a city composed of numerous islands and the only ways between them were by tunnel or bridge or ferry, then gangs that controlled entire islands would only need to watch the points of entry, and defining boundaries between gangs, through the waterways between islands, would be pointless.

$\begingroup$Great point about Start Trek sensors. The ability to see into an area compels you to claim and defend it, or at least watch it.$\endgroup$
– fredsbendAug 8 at 19:44

$\begingroup$It's not just being able to see into an area, it's being able to go into an area. If you don't have FTL at will wherever you need it, then the space between is, for all intents and purposes, utterly irrelevant. In the Starfire novels for instance, the "map" everyone used in determining who controlled what was based on the point-to-point travel routes, which had almost nothing to do with the actual position of those stars in space. Trying to map who owned what in real space would have looked like an insane tangle of different colours of yarn.$\endgroup$
– Keith MorrisonAug 8 at 21:27

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$\begingroup$Being able to scan an area and not go into it just makes the irrelevance of defending it even more pointed. "Admiral! We've detected an enemy fleet approaching in realspace 5 light years out! They'll get here in...um, 8 years." "Well, leave a note for my successor. I'm due to retire in three."$\endgroup$
– Keith MorrisonAug 8 at 21:30

In a planetary system, at least, there are valuable places in open space, starting with orbits (orbits that intersect my own are either threatening, valuable, or both) co-moving regions of space (like Lagrange points) or simply lines of sight (for solar power, perhaps, or communication). These places could be tens or hundreds of millions of kilomtres away from habitation, and have nothing there but a slightly higher concentration of dust and gas than the rest of the solar system, but their economic or militaries values could be vast.

None of those apply in interstellar space, which as the good book says, is BIG. At that point, where you search and patrol and protect will be heavily influenced by your transport, communications, sensing and weaponry technology and as such the question is a) super wide open and b) totally subject to the whims of the fictional universe's author.

I will start by saying the Star Trek idea of parceling up space is unrealistic. The short answer is that there is not a very good reason for closing off empty space. In reality, "open space" probably would work more like "international waters," in that a certain distance from someone's home planet is considered territorial, but beyond that it's essentially the law of the high seas (or in this case, space?). Space is so vast this would likely even happen on a planetary level, rather than a star system level (i.e. space between Earth and Mars is international space).

However, that wasn't your question so I will work within its bounds.

The main reason you would close off space is to control lines of approach. This works especially well in the linear FTL world of Star Trek and Star Wars. You can't "cross my space" because it allows you to get close to something of value, like my home world. In this case, space acts more like a buffer. This way, it makes it much more difficult for someone to explain why they're in "your space," as they would have no reason to be there unless they were traveling towards your home world or a world you've claimed. However, even this doesn't quite fit, as countries aren't allowed to close off large portions of international waters "just because" it provides a straight line to their shores, and in some cases there are bodies of water recognized as "important" where territorial restrictions don't apply. Generally this is in cases where the body of water is the only access to an area (such as the straights of Gibraltar, the Bosphorus, Straights of Hormuz, etc...).

Given this, it becomes difficult to justify parceling off space, so the next remaining possibility is one of resources.

Empty space isn't empty at all. Even interstellar space is likely to contain asteroid and planet-sized rogue bodies. You could, in this case, say "all the stuff in this volume of space is mine, whether I know about it or not." In this case, it becomes something akin to a country's Exclusive Economic Zone, which is much larger than territorial waters (200 miles vs 12 miles). But even in the case of an EEZ, international law permits any and all innocent passage through its waters, including that of military ships, so once again it becomes impossible to justify whole blocks of space being off-limits to ships.

With FTL travel though, it becomes possible for a ship to arrive at a destination before anyone has the opportunity to detect it. In Star Trek, this isn't the case because subspace sensors can detect FTL ships, but in reality it would be very hard to detect a ship at warp. With this, you can again make the buffer case, and say that no ships should be allowed to cross into my predefined box because they can get to my planets before I have a chance to respond. This is the one difference between seagoing navies and spacegoing FTL navies. If I send a flotilla of ships to your country through international waters, you have days, weeks even to act and prepare yourself. If I send an FTL ship to your planet, it could theoretically arrive there in minutes, making it more akin to an aircraft problem than a sea vessel problem.

In this case, rather than being like shorelines and international waters, your space territory becomes one of airspace, and airspaces must be relatively well-controlled. This allows you to make the case that any starship entering your space territory must be under the control of the local space traffic authorities, and a warship refusing to follow these protocols is one that is acting in a dangerous manner, which could be considered an act of war.

So to answer your question: The best way for this to work is to treat your space territory as an airspace, requiring that FTL ships be in constant contact with control authorities to ensure safe passage for all ships. After all, two ships colliding at FTL speeds would release an immense amount of energy that would likely create a black hole (Ek = 0.5mv2 after all, and velocities greater than the speed of light imply incredible amounts of energy involved). Any ships refusing to operate under local controls are thus a navigation hazard and should be treated as a hostile actor.

Your space-nation would likely need to organize these FTL airspaces into controllable lanes, ensuring proper spacing between ships in an FTL lane and safe distances between overlapping lanes traveling in different directions or at different speeds. In Earth airspace, there is a concept of uncontrolled airspaces, but in your FTL lanes, any FTL speed is likely to require control, so uncontrolled airspaces might be relegated to sublight speeds. Earth airspaces require 1000 feet of separation in altitude, and usually 30 miles of separation for aircraft at the same altitude. You'd probably want to increase your boundaries based on maximum FTL speeds allowed in a given lane, and have them set so that a human-equivalent-alien can react in time to avoid a collision at the speed given. Let's set this at no more than 30 seconds. Traveling Warp 2 in the Star Trek universe is 10x the speed of light, so at this speed your separation distance would have to be about 90 MILLION kilometers, or about half the distance between the Earth and the Sun. 0.5 AU sounds big, and it is, but in interstellar space you could have thousands of FTL lanes easily. You would probably want a central set of high-speed corridors with no-go zone buffers in between, then you can surround that with slower speed corridors, etc...

Given the dangers involved, there would likely be FTL no-go zones near planets to prevent accidents, and given that space gets less empty the closer you are to a star, it's likely the FTL/sublight boundary would be at the far edge of a solar system. You sometimes see this plot-hole come up in Star Trek, where actors will say things like "you want to go to warp inside a solar system?!" or otherwise imply that going to warp near large bodies is unsafe and undesirable.

Truly deep, interstellar space would likely be nothing but FTL lanes, as there's almost no reason someone would want to be going at sublight speeds there, but you would probably want "step down" zones the closer you got to stars, allowing ships to slow to lower warp levels and finally to sublight speeds as they get close to planets.

This is a little off in the woods from your original question, but all of this regulation takes up a lot of space, time, and money. It goes to reason then that each of the nations undertaking the maintenance and control of the FTL lanes in their space has a vested reason to claim sovereignty over that space, and that any ship operating outside of the rules established by the local sovereign is a threat to the local order, thus you have a reason for the boundary lines specifying ownership and governance.

$\begingroup$This answer seemed to be going in the direction of explaining how airspace works on Earth, defining sizes like you did with water, but then leaves the reader wanting more when those numbers aren't given.$\endgroup$
– MuuskiAug 8 at 17:03

$\begingroup$@Muuski Apologies that you think it incomplete, but if the reader is left wanting more I guess it was at least the start of a good answer. ;) I'll try and fill it out a bit, but airspace management is extremely complex (waterspace management is as well, but orders of magnitude less so).$\endgroup$
– stixAug 8 at 17:09

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$\begingroup$I think this is a great answer. I was thinking almost exclusively militarily and strategically. You make a good argument for "space traffic control" and rightly point that bad actors are easily found because they break simple rules. I suppose the amount of traffic control exercised would be a function of traffic volume and proximity to valuables, and FTL lanes themselves can become valuables over time. A space version of "off road" seems likely, considering the vast sizes. Great answer!$\endgroup$
– fredsbendAug 8 at 19:26

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$\begingroup$@fredsbend (1/2) It can be expanded even further, but it's already getting a bit outside of the context of the question. FTL lanes would need to be relatively straight, since turning at greater than the speed of light, even in long arcs, would be crazy due to any number of problems, not the least of which are things like Bremstrallung and reaction time. The lanes would also have to be routed around gravity sources, further resulting in chokepoints and constrictions, and so for trade a space nation would want to control a lot of contiguous FTL lanes to ensure rapid transit.$\endgroup$
– stixAug 8 at 19:38

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$\begingroup$(2/2) These requirements on contiguous and straight FTL lanes would drive nations to form their space claims in large, contiguous blocks, which is the perfect environment for requiring a border. The fact that you could cut a space nation in half by disrupting its FTL lanes, and thus impede the flow of goods and damage its economy, would further require military protection of their claims. All of this very quickly becomes something we know very well: "territory."$\endgroup$
– stixAug 8 at 19:41

Besides all the other answers, there's the concept of trade routes. If your shipping vessels have to physically traverse space between destinations, you will want to have these trade routes.

Space isn't a complete waste of nothingness. There's plenty of things like black holes, various stars with dangerous radiation, hostile alien races, and more. Even in space, the quickest route between 2 locations is a straight line. Unfortunately, if you have a black hole directly between those two places, you'll want to go around it. You may also want to go around solar systems, since the space there can be a bit crowded and dirty with space debris when flying at "warp X". Yes, you can go "over" or "under" the objects as well, but sometimes a detour to a nicely exotic planet for shore leave works, too.

Maintaining these trade routes is key. There may be roving bands of pirates or scavengers which need to be defended against, but there also may be comets/meteors that need to be noted on maps. Keeping track of all this is part of "defending your territory".

If you can't move freely around, then it's not your territory and it's someone else's. They may set up tolls to cross the volume or simply commandeer the vessels, cargo, and personnel. They may just blow your ships up, without warning. These are all bad things when it comes to trade. You can't make money when your ships keep getting destroyed. You can feed or medicate your people if it's getting re-routed to another planet or system. You can't even go back to continue exploring an interesting spacial anomaly, planet, star, or whatever if someone else has decided it's "their's".

Setting up buoys in space, away from the trade route for an extra margin of safety, saying that this space belongs to you, having patrols, or even having a treaty with neighboring systems and it's just a line/plane/whatever on a star map are ways to make sure your ships are safe.

Trade routes are important for more than just your ships, too. Other civilizations might want to use them, potentially paying you money, favors, or items you need or want. Having a known and safe path means less cost and fewer problems for a shipping company and any governments involved.

I think you have an underlying problem here: how do you define your borders?

On Earth, stating "the waters within 100 km from my coast are my economically exclusive zone" makes sense, because your coasts do not move with respect to the others (excluding tectonic movements).

In space stars and planet have different relative motion, so it will happen that borders will cross each other just because the reference points are moving with respect to each other.

That aside, space is empty, but has one valuable item: energy, in form of electromagnetic radiation (and dark matter if you can use it). If you put energy harvesting means in space, you don't want anybody else to cover them or to harvest the energy of your volume of space.

$\begingroup$Dealing with the problem of your star's EEZ colliding with someone else's star's EEZ can be safely left to your distant descendants. There's no danger of it happening unexpectedly.$\endgroup$
– Starfish PrimeAug 8 at 8:31

$\begingroup$I would imagine that star faring civilisations would have the concept of "relative borders". Just like Iceland. It's about 8 feet (3m) wider than it was a century ago (and probably 100 feet wider since the Althing first met in 930). It's maritime zone simply grows with it. As star systems move relative to one another, the borders of star empires move and change shape. In real terms, quite a lot faster than Iceland's borders! This could become a real issue if, e.g., a valuable relatively fast moving system begins to "invade" the relatively slower trailing edge of a rival's border...$\endgroup$
– elemtilasAug 8 at 16:07

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$\begingroup$Our own solar system is moving pretty quickly, and in a different direction to most nearby stars. So this is not entirely an academic point even in the real world; every so often, some other stellar object comes close enough to gravitationally perturb objects in solar orbit. This is most likely why comets exist.$\endgroup$
– ChromatixAug 8 at 20:10

$\begingroup$@Chromatix Its only an issue over thousands of years, and it is likely that borders would be reshaped by either diplomacy or, failing that, wars over the course of that time. Even in Star Trek, the peace-loving federation is constantly involved in armed conflicts like the war with the Klingons, Romulans or Dominion.$\endgroup$
– PolygnomeAug 9 at 18:34

The other answers detail why you want to have well-defined borders, but are a bit short on how you actually control your space.

It all depends on the technology available. and you as the author can decide that.

First, if FTL is instantaneous, there is no point. Dune model applies.
If FTL takes time, but is undetectable until arrival, much the same applies. (Think submarines)

Interesting things happen when FTL takes time and there are ways of detecting it.

As we all know, Space is Big. You can adjust how Big space is by adjusting the range of your detectors. As the range starts passing tens of light years, space isn't so big anymore.

On the other hand, how far apart are useful star systems? If you have to travel hundreds of light years to get anywhere interesting, space starts looking big again.

The bigness of space depends on the relation between those two numbers.

The next question is how fast the detector sees things. If someone is moving FTL a light year away, when will the detector beep? Instantly, one year from now, or something in between?

Then there is the speed of communication. A detector has beeped. How fast can it alert the proper authorities? If you want a coordinated military response, communication had better be fast.

Where do you place the detectors? Obviously you want them near your colonies and on your ships. A colony detector can be bigger and more powerful than a ship detector. The ship detector is also likely to be disturbed by the ships own drive.

In many cases it will also make sense to spread out "detector buoys" which is a simple detector plus FTL communicator combination. They can't stop an attack, but they can scream before they die.

A universe or galaxy with only one known polity, then I think your observations are very accurate. Under those conditions, occupying space would be useful for purposes of rendering aid to travelers between stars. Having a maintainable presence within some defined response time of a ship suffering a disabling casualty would be benefit society economically and socially by encouraging interstellar travel, communications and trade.

If there are pirates then it becomes even more important and the rescue vessels would armed.

If the galaxy has multiple polities and they are hostile, then I think you can add another reason for occupying space is provide a deterrent. To position forces that can interdict attacks or pose a threat to the opposing side that they can’t risk attacking. The Fulga Gap and the Golan Heights would be terrestrial equivalents of this idea.

$\begingroup$Naturally, if there's an enemy, you want every advantage you can get. But what does it take, besides the desire, to actually occupy space?$\endgroup$
– fredsbendAug 8 at 20:10

$\begingroup$@fredsbend, I guess the answer to your question is to project force that controls who can move in or across, or act in that sphere of space. Its not very different from how militaries today want to control land, air, or sea, and soon space$\endgroup$
– EDLAug 8 at 20:40